German Patent No. 2304464 describes a probe in which a gold or silver electrode which does not catalyze establishment of equilibrium in the gas mixture and works in conjunction with a platinum electrode that does catalyze establishment of equilibrium in the measured gas is provided. The catalytically inactive electrode materials cause a competing reaction between the oxygen and the oxidizable and, respectively, reducible gas components to take place at that electrode. Even if adjustments have been made to ensure high lambda values, very little of the free oxygen that is conveyed along with the measured gas reacts with, for example, C3H6 or CO; as a result, free oxygen as well as C3H6 and, respectively, CO reach the three-phase boundary at the catalytically inactive electrode (non-equilibrium state).
A gas sensor having a measuring electrode and a reference electrode arranged on a solid electrolyte is described in European Patent #466020. In order to create a mixed potential electrode, the measuring electrode is made of a platinum compound or a ternary alloy that includes platinum, gold, nickel, copper, rhodium, ruthenium, palladium or titanium. Herein, the materials may be applied to the solid electrolyte as multiple layers, the alloying step being carried out after the materials are applied.
U.S. Pat. No. 4,199,425 describes a gas sensor in which a platinum electrode covered by a porous protective coating is provided. The pores of the protective coating are impregnated with a further catalytic material, rhodium. The rhodium renders the gas sensor sensitive to NOx as well as oxygen. Herein, the rhodium coats the walls of the pores of the entire protective coating; as a result it is impossible to specify the thickness of the layer in the porous protective coating.
The gas sensor according to the present invention having the characterizing features set forth in claim 1 has the following advantage: a sintered sensor element basic body can be used, the further layer being integrated via just one additional deposition step following the sintering. As a result, the outer electrode of the sensor element basic body can be modified following the sintering. The sensor element of a Nernst-type lambda sensor, for example, can be used as the sensor element basic body, it being possible to transform the outer electrode into a mixed potential electrode by making certain modifications. Furthermore, it is advantageous that materials that would not withstand the high temperature at which the sintering is carried out can be used as the further layers. A further advantage is that the further layer system, which is directly adjacent to the electrically conductive base layer, does not completely fill the pores of the porous protective coating. As a result, the porous protective coating continues to provide effective protection, and sufficient gas can access the three-phase boundary. Herein, the material used as the further layer may be used to modify the functional characteristics of the electrode of the gas sensor in a specific manner. Herein, this modification may define the specific gas selectivity of the sensor and/or its position within the control system.
Advantageous further refinements of the gas sensor according to the present invention and the method according to the present invention can be achieved via the measures set forth in the subordinate claims. A particularly advantageous sensor designed for mixed potentials can be achieved if the layer system is subjected to a thermal additional treatment following deposition of the further layer. For example, in the case of a Pt/Au electrode a temperature range of 1200xc2x0 C.xc2x1100xc2x0 C. is favorable. At this temperature, the metal atoms of the further layer diffuse into the metal of the adjacent base layer. A further advantage is that a cermet layer is used as the electrically conductive base layer which, thanks to its ceramic component, creates a solid join with the solid electrolyte when the ceramic body is sintered. Furthermore, by creating a plurality of further layers and choosing the layer material appropriately one can specify the selectivity and also modify the catalytic activity of the electrode with even greater precision.